Here’s a nice summary of the evidence for magnesium in migraine control by Steve Novella, neurologist, scientist and skeptic:
"Magnesium for migraine is a plausible hypothesis. Magnesium can affect both regulation of blood flow and neuronal function – both of which are physiological factors important in migraine. There is evidence that magnesium deficiency is common, and is more common in migraine patients than non-migraine sufferers.
The use of magnesium in migraine is a good example of how a science-based practitioner might incorporate benign and plausible treatments but with only preliminary evidence into their practice."